Physicists create ultracold atomic bubbles in space!

Since the first experimental achievement of the Bose-Einstein condensate (BEC) in 1995—fully 70 years after it was predicted by Einstein and Bose—ultracold atomic physics has generated intense interest for its potential to answer fundamental questions in related fields, especially condensed matter physics, astrophysics, and quantum information science. 

Now, a multi-institutional team of scientists has reached a new milestone by reporting the first experimental observations of ultracold atomic bubbles in a microgravity environment in a paper published online on May 18, 2022, in the journal Nature. The experiment, conducted in the NASA Jet Propulsion Laboratory’s Cold Atom Laboratory (CAL) on the International Space Station (ISS), continues theoretical work initiated two decades ago by Illinois Physics Professor Smitha Vishveshwara and colleagues. In the current research, Vishveshwara joined forces with a team of physicists from Bates College (led by experimentalist and Bates Physics Professor Nathan Lundblad), NASA's Jet Propulsion Lab at the California Institute of Technology, Smith College, and the University of Massachusetts Amherst.

Predicting behaviors of shell-shaped condensates 

Over the last 30 years, studies in ultracold physics have led to fundamental discoveries in superconductivity, magnetism, and even the emergent field of quantum computing. Scientists can control atoms with exceptional precision by trapping and cooling them down to billionths of degrees above absolute zero, the point at which atomic motion ceases.

Vishveshwara explains, “Experimentalists can create an optical lattice of interfering laser beams that trap atoms at extremely cold temperatures. The resulting system is very tractable: you can control the strength of the trap and the interactions between atoms.”

This process creates quantum landscapes in some of the coldest spaces in the universe, enabling the simulation and study of a wide variety of systems. Such systems range from materials that conduct electricity with zero resistance, called superconductors, to neutron stars, and even cosmological inflation.

Among the coolest states of matter explored through ultracold atomic physics is the BEC. When bosons—one of the two flavors of quantum particles—are cooled down to ultracold temperatures where quantum mechanics dominate, the bosons act like waves. At a sufficiently low transition temperature, the waves overlap and fall into the same state. Physicists say they “condense,” behaving like a single, collective superparticle: the BEC. This state of matter often behaves like a superfluid, flowing with zero viscosity. Both BECs and superfluids occupy prominent places in Vishveshwara’s theoretical research.

In the early 2000s when Vishveshwara was a postdoctoral scholar at Illinois Physics, she and theory colleagues began studying ultracold systems having shell-shaped geometries, in collaboration with cold atom experiment pioneer and Illinois Physics Professor Brian DeMarco. These structures arise when ultracold atoms are rigidly held in place by an optical lattice and then allowed to probabilistically jump to neighboring lattice sites, a process known as quantum tunneling. The resulting system forms concentric shells of two distinct states of matter.

“One state is an insulating state, where atoms are pinned to the lattice sites,” explains Vishveshwara. “A family of these states can form concentric insulating shells, and when enough quantum tunneling is allowed, a second, superfluid state can arise between the shells.”

The upshot is that one obtains superfluid shells sandwiched between insulating regions, and these structures have been observed in optical lattice systems.

The superfluid shells are interesting in their own right: they possess unique rotational vortex states that may give insight into fundamental quantum physics. Moreover, there are many compelling  theoretical applications for this work, including some astrophysical theories predicting that similar superfluid shells form in neutron stars.

Vishveshwara began focusing on the theory behind these shells in isolation—shells devoid of the insulating regions—in collaboration with a colleague from graduate school, Courtney Lannert of Smith College and the University of Massachusetts Amherst, and with former Illinois Physics postdoctoral researcher Roman Barankov and Illinois Physics graduate student Tzu-Chieh Wei. Together they studied the shells’ superfluid phases in detail—their energy states, how they interact, and more.

Experimental realizations of these isolated structures were impossible at the time because of the influence of terrestrial gravity, which distorts the atomic ensemble.

Vishveshwara explains, “In real life, there’s gravitational sag, which distorts the desired hollow shell-shaped structures into a contact lens shape, so the shells can’t form. Just a fraction of a percent of terrestrial gravity is enough to ruin the shells.”

Cold Atom Laboratory on the International Space Station enables microgravity experiments

Fast-forward more than a decade. Experimentalist Nathan Lundblad of Bates College was on the hunt for theory-driven work on shell-shaped BECs, in anticipation of the 2018 installation of the CAL on the ISS.

CAL is a remotely operated, multi-user facility, the first of its kind to enable studies of quantum gasses in a non-terrestrial, microgravity environment. The principal apparatus is a vacuum chamber equipped with trapping and cooling tools required for BEC. Atoms are loaded into a specialized chip that creates the right conditions for condensation.

Lundblad recalls, “When NASA put out a call for proposals in 2014 for prospective experiments on CAL, they explicitly requested that the proposed science require microgravity. People had been making sagged, incomplete bubbles on Earth for a while, and I immediately thought that CAL could be a home for bubble potentials.”

Vishveshwara and Lannert were among the few theorists in the world who had studied these structures. Lundblad approached them and proposed partnering up in a study of very thin, hollow, fully closed shells, or “bubbles” in microgravity.

Vishveshwara and Lannert continued their work on shell condensates, this time with former Illinois Physics graduate students Kuei Sun and Karmela Padavic-Callaghan. They theoretically studied the statics and dynamics of shell-shaped BECs as well as their vortex behaviors in both two- and three-dimensions. Then, in joint work with Lundblad and Vishveshwara’s current graduate student Brendan Rhyno, they worked out the thermodynamic properties of shell-shaped BECs in spherical traps, ones that atomic ensembles would experience aboard the ISS.

Vishveshwara notes, “Motivated by the expected realization of hollow condensates by the space-based CAL, we began studying the collective mode behaviors of shell-shaped structures. For example, we studied what changes we might observe when moving from a filled sphere of atoms to a hollow sphere, two topologically different structures.”

At the same time, Lundblad’s group conceived of experiments based on a special trap originally proposed by Zobay and Garroway. This trap uses radio frequencies to shape collections of atoms into the desired bubbles.

Creating bubbles in microgravity

When the experiment was finally run at CAL, the ultracold ensemble of approximately one billion bosonic rubidium-87 atoms, confined in the specialized radio frequency trap, coalesced into bubbles, revealing shell structures, as predicted..

By bathing the bubbles in increasingly energetic radio frequencies, the researchers could “inflate” the bubbles, making them larger. They inflated the delicate bubbles slowly enough to minimize heat production, a process called adiabatic inflation. (An adiabatic inflation increases the likelihood that the bubble enters a condensed state.) The bubbles reached sizes of several tenths of millimeters—large enough to see with the naked eye—and thicknesses of several micrometers at temperatures below 600 nanokelvin.

“What's neat about CAL is that it's a user facility that we don't have total control over,” Lundblad notes. “CAL ‘delivers’ a ready-made BEC to us and helps us design sequences using a default BEC as a starting point. We then take that trapped BEC and deform it in such a way that, when filled with atoms, yields a bubble-like ensemble.”

The authors also documented the bubbles’ unique thermal profiles after the trap was turned off by allowing the bubbles to expand and imaging their resulting clouds. While adiabatic expansion results in temperature drops, structures that began uncondensed remained so because the transition temperature too drops and the current capabilities cannot achieve condensation for the largest observed bubbles. In fact, while the coldest bubbles initially lay close to the condensation transition temperature, upon expansion, they became uncondensed.

Rhyno, who performed numerical calculations of the bubble thermodynamics, notes, “Most of the samples started off uncondensed but still ultracold. In these cases, we generally saw good agreement between our model and the experimental data.”

The researchers also observed some discrepancies.

Lannert explains, “For the largest systems that start at the coldest temperatures, there were some differences between the predicted and measured temperatures during expansion. These differences may signal that the bubble expansion is not happening adiabatically.”

What’s next? Condensed bubbles!

The researchers are already turning their attention to future studies of ultracold bubbles.

“I'm excited for the possibility of achieving a truly condensed bubble,” Rhyno says. “The experiment was tantalizingly close to condensation, but I hope future runs will be able to generate a fundamentally quantum phase.”

Future experiments will need to prevent heat generation to ensure adiabatic expansion and keep the bubbles in the condensed phase. The researchers predict a whole suite of new physics unfolding once this is achieved, including superfluid vortices and collective excitations.

The researchers also hope to resolve other shortcomings, including distortions in the bubble images, as well as potential losses of condensate. 

“There are planned upgrades aboard the ISS that should get us closer to ideal trapping potentials and even colder temperatures so that the bubbles will stay condensed as they’re inflated,” Lannert notes. “Also, an improved imaging system will image the bubbles’ densities from other directions.”

Lannert adds, “In many ways the physics of hollow condensates is still in its infancy, and I'm sure over the next five years and beyond we'll see more people getting interested and making predictions for really cool behavior that we haven't even imagined yet.”

Vishveshwara concludes, “It’s serendipity, how we happened to be looking at these shells in a completely different context, namely, the insulating shell systems. We were so lucky that Nathan found Courtney and me. And now, in this second phase of our work, our ideas have gone spaceborne. They’re no longer just in theorists’ fancies, but a part of these fantastic experiments, which contain richer mysteries and prospects than anyone originally imagined. We eagerly await what comes next!"

 

Designed, managed and operated by Jet Propulsion Laboratory, the Cold Atom Lab is sponsored by the Biological and Physical Sciences Division of NASA’s Science Mission Directorate at the agency’s headquarters in Washington and the International Space Station Program at NASA’s Johnson Space Center in Houston.